1. Field
One embodiment of the invention relates to a method of refreshing data stored on, for example, a disk.
2. Description of the Related Art
In recent years, with the increasing storage capacity of magnetic disk drives, the recording density and track density have been getting higher. As the track density gets higher, the distance (i.e., track pitch) between adjacent tracks (or data tracks) on the disk gets shorter. Each track has the same width as that of the head (or a writing element included in the head). However, the width of the distribution of the recording magnetic field produced by the head in a write operation does not necessarily coincide with the width of the head, with the result that the magnetic field is also applied to the surrounding area. This state is known as write fringing.
If the track pitch is made narrower, there is a possibility that the data (or recorded data) written on adjacent tracks will deteriorate in writing data onto the tracks. The deterioration of data is attributable to write fringing and errors in positioning the head on the track. If the recorded data is deteriorated repeatedly, it becomes very difficult to read the data. That is, even if an attempt is made to restore the recorded data by making best use of an error-correcting code (ECC), it is difficult to restore the data.
With this backdrop, in the field of recent magnetic disk drives, data refreshing (or rewriting) is indispensable for restoring the recorded data before the data becomes unreadable due to data deterioration. As a method of returning the recorded data to the normal state, an operation called data refreshing exists, in which the deteriorated recorded data is read from, and then rewritten to a track.
For example, Jpn. Pat. Appln. KOKAI Publication No. 2004-273060 has disclosed a technique for refreshing the data written on a track adjacent to a track whose number of data writes has reached a specific value (hereinafter, referred to as the related art). According to the related art, first, it is determined that the data on a track adjacent to a track whose number of data writes has reached a specific value has deteriorated. Next, the data on such an adjacent track is read as data to be refreshed and is stored temporarily into a RAM. Then, the data on the adjacent track temporarily stored in the RAM is written again onto the adjacent track. That is, the data on a track to be refreshed is rewritten with the data read from the track. By such a data rewriting operation, or a data refreshing operation, the data is restored from the deterioration.
In the related art, however, if the power supply of the magnetic disk drive is shut down during a data refreshing operation, the data on a track to be refreshed can be lost. More specifically, if the power supply of the magnetic disk drive is shut down during the operation of writing the data read from a track to be refreshed onto the track again (hereinafter, referred to as a rewrite operation), the data can be lost. The reason why this is so will be explained below.
First, suppose the power supply is shut down during a rewrite operation. In this case, the data on a track to be refreshed is destroyed. At this time, the data on the track to be refreshed temporarily stored in the RAM disappears. Accordingly, even if the power supply is restored, the unfinished rewrite operation cannot be performed again, with the result that the data on the track to be refreshed is lost.
To prevent such a problem, use of a specific track on the disk in place of the RAM is considered. That is, use of a specific track on the disk as a save area (or a backup track) for temporarily saving (i.e., backing up) the data on the track to be refreshed is considered.
When a specific track on the disk is used as a save area, a save operation and a rewrite operation are carried out as follows. In a save operation, the data read from a track to be refreshed is written onto (or saved to) a specific track. In a rewrite operation, the data read from a specific track is rewritten onto the track to be refreshed, with the result that the data recorded on the track to be refreshed is refreshed.
In a save operation, a first seek operation is performed to move the head from the track to be refreshed to a specific track. In a rewrite operation, a second seek operation is performed to move the head from the specific track to the track to be refreshed.
As described above, when a specific track is used as a save area, two seek operations are required to refresh the data on a single track. Generally, recent disks have about 160,000 tracks per recording surface. In a magnetic disk drive with such a disk, the head has to move over a long distance corresponding to an average of 160,000 tracks to refresh the data on a single track.